Ultra-thin slab or thick-strip casting

ABSTRACT

A system of twin-roll casting an ultra-thin slab or thick-strip of metal with a liquid core still present inside the hollow shell of the casting as it exits the nip of the opposing counter rotating casting rolls. The wide sides of the cast shell are formed against a recessed casting surface on each roll and the narrow end walls of the shell are formed against the tapered ends of the recessed area so one half of the narrow end wall is formed on each roll and the two halves are joined together at the nip to form the full perimeter of the metal casting. A liquid-cooled copper mold liner provides continued support to each of the four sides of the casting below the twin-rolls and contact with those chilled copper mold liners continues to remove heat from the liquid core of the casting enabling the shell to continue growing in thickness. As the casting exits the mold it will be of adequate thickness to pass through opposing pairs of support rolls below the mold without bulging where water sprays can provide the additional cooling needed for final and complete solidification. A variation of this system uses one or more separating dams to cast different metals or different alloys of the same metal to integrally cast a dual-layer or multi-layer ultra-thin slab or thick strip of metal with a thin metal layer on one or both of the wide and narrow end casting surfaces.

This is a nonprovisional of U.S. Provisional Patent Application 61/408,736, filed Nov. 1, 2010, the entirety of which is hereby incorporated by reference as if set forth fully herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application relates to the field of high speed, high production metals casting and more specifically to twin-roll casting of an ultra-thin slab or thick strip of solidifying metal with a liquid core remaining beyond the nip of the twin casting rolls. This application further relates to a variation of this technology to integrally cast a dual-layer or multi-layer ultra-thin slab or thick strip with one or more surface claddings of a different metal or different alloy of the same metal on one or both sides of the casting.

2. Description of the Related Technology

In the field of continuous casting of metals several relatively new methods have been developed to cast a shape that is near that of the final net product being produced. “Near-net-shape” casting of a coiled flat product has evolved from casting a thick slab that requires a significant amount of reduction by hot rolling to casting a thin slab that requires less rolling, thus saving significant amounts of energy. For coiled materials being rolled to thicknesses less than a few millimeters, even thin-slabs requires more rolling energy than necessary to reach the net thickness. Thus thin-strip casting by the twin-roll casting process is being pursued for very thin metal production.

Another process called “thin-strip” casting is being pursued and promoted as offering “Energy savings of up to 2.4 million British thermal units (Btu) per tonne of steel produced” according to the U.S. Department of Energy fact sheet entitled “Strip Casting: Anticipating New Routes to Steel Sheet”. However this casting method produces only very thin sheets of steel up to 3 mm thick. In addition, the throughput from a thin-strip caster is less than one third that of a thin-slab caster. Because the product is so thin to begin with, there is no chance of achieving the necessary reduction ratios needed for many high-quality steel grades.

The drawback of near-net-shape casting of thin strip is low production throughput. Whereas a thick slab caster can produce as many as 3 million tons of steel coils annually on a single strand and a thin-slab caster can produce as many as 1.8 million tons annually on a single strand, a thin-strip caster generally produces less than 0.6 million tons annually on a single strand. The thin-strip casting limitation that prevents higher production rates is the need for the cast metal to be fully solidified by the time it leaves the nip or closest point between opposing rollers used in the twin-roll casting process. If not solidified by then the liquid metal rushes out of the pool above the nip and causes the strand shell to bulge below, which can lead to a rupture or breakout as it is called in the continuous casting industry. Accordingly, the steel industry currently has a huge void when it comes to continuously casting steel in the thickness range between 3 mm (⅛ inch) thick and 2-inch thick. As a result the original intent to reduce energy by rolling thinner slabs has been all-but-forgotten because of the desire for greater throughputs.

In order to improve the productivity of the twin-roll casting process, a means of strand shell support below the rollers is needed to prevent bulging normally caused by the ferrostatic pressures within a newly formed shell with a liquid metal core remaining after the roll nip. With such strand support, the twin-roll spacing could be made wider to produce a thicker casting that would exit the roller section with a solidified outer shell and a liquid core of molten metal inside that would continue solidifying as it travels through the support sections below. Such a post-roll shell support system is described in PCT International Publication Number WO 96/01710 dated 25 Jan. 1996. It cites, “Immediately downstream of the twin rolls, the cast strand is cooled by directing it through a stationary cooled mold”. The publication further describes a preferred casting thickness of 5 to 35 mm and a steel throughput rate of 1 to 6 tonnes per minute. That would make the process a hybrid between a thin slab normally 40-90 mm thick and a thin-strip normally 1-5 mm thick caster yet still having a maximum productivity rate somewhere between thin-slab casting 1.8 million tons annually and thick-slab casting 3 million tons annually.

What the casting process described in WO 96/01710 fails to describe is a system for solidifying and supporting the narrow ends of the cast product to prevent leakage of the molten metal out through the narrow ends during casting. Without such means the liquid metal would spill out of the casting as soon as it exits what are described as the “side dams 83” causing interruption of the cast and damaging the equipment below. Since side dams, or end dams as other twin-roll operations call them, are generally made of a refractory material that does not function well as a heat exchanger to promote solidification, one can imagine that the narrow ends of the cast product would still be molten or if they did solidify at all they would be V-shaped and would crumple as they are pulled through the nip between the opposing rolls.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a system and method that will form one wide side and half of each end wall of the cast shell on each twin roller so those halves will join together as the casting exits the twin-roll section at the nip in order to complete the outer perimeter of the metal casting.

It is further an object of the invention to provide water cooled support for both the wide sides and the narrow end walls in the mold below the roller nip to prevent metal leakage from the seam between the narrow end halves and to promote continued solidification of the product until it exits the mold section or until such time it has adequate thickness to support itself without bulging between support rolls below the mold.

It is also an object of one aspect of this invention to simultaneous cast two or three different metals or metal alloys by positioning separating dams over one or both of the rolls so the initial solidification on one or both roller surfaces is of one or another metal or metal alloy and after it passes by the tip of the separating dam subsequent solidification occurs from a different liquid metal pool of another metal or metal alloy thus forming a multilayer metallic structure.

In order to achieve the above and other objects of the invention, a twin roll continuous casting system according to a first aspect of the invention includes first and second casting rolls; and a recessed roller surface on at least one of the first and second casting rolls for forming at least a portion of a casting shell.

A method of continuously casting a metal material according to a second aspect of the invention includes steps of providing a molten metal material; and cooling the molten metal material into a solid casting using a casting roller that has a recess defined therein.

A twin roll continuous casting system according to a third aspect of the invention includes first and second casting rolls that are constructed and arranged to form a casting shell; and support structure positioned beneath the casting rolls for supporting substantially an entire perimeter of the casting shell as viewed in transverse cross-section.

A twin roll continuous casting system according to a fourth aspect of the invention includes a cavity that is constructed and arranged to hold a liquid metal pool; first and second casting rolls; at least two separating dams positioned axially within the cavity for separating the cavity into at least three separate metal pools; wherein different materials may be cast simultaneously in at least three layers into a single casting shell.

A twin roll continuous casting system according to a fifth aspect of the invention includes a cavity that is constructed and arranged to hold a liquid metal pool; at least one separating dam positioned within the cavity for separating the cavity into at least two separate metal pools of different materials; and first and second casting rolls for receiving material from the at least two separate metal pools and casting at least a first layer having a first thickness and a second layer having a second thickness into a single casting shell, and wherein a ratio of the first and second thicknesses is substantially within a range of about 1:2 to about 1:40.

These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical depiction of a system for casting an ultra-thin slab or thick-strip using the twin-roll casting process with the wide side casting surface of each roller recessed into the outer roll diameter to a depth equal to one half of the cast thickness. The phantom line on each roll represents the depth of that wide side casting surface on each roll, which is further illustrated by the corresponding thickness of the cast slab exiting from the bottom being equal to the distance between the recessed surfaces of the two rolls.

FIG. 2 is a diagrammatical depiction of that system as viewed from the center looking toward one end. In this view the wide side casting surface can be seen with a solidifying surface that begins at the uppermost location of the molten metal pool continuing to grow in thickness toward the nip and beyond as the casting loses heat first to the cooled roll and afterwards to the cooled mold below. This view also depicts the liquid core in the center of the casting that has not yet solidified at the bottom extremity of this view.

FIG. 3 is a diagrammatical depiction of that system as viewed with the liquid metal removed to illustrate solidification of the metal against the end walls of the recessed area and the joining of the two halves of the casting at the roll nip.

FIG. 4 is a diagrammatical depiction of the ultra-thin slab or thick-strip shape as it exits the twin-roll section of the caster. This view illustrates the radius on each corner of the cast slab and the angle formed by the junction between the wide side casting surface and the end wall casting surface. This view further illustrates the joint where each half of the end walls meets to complete the outer perimeter of the metal casting.

FIG. 5 is a diagrammatical depiction of the casting system viewed from above to illustrate the liquid metal pool in the center and how it contacts the end wall sections of each roller to begin solidification of the end walls simultaneously with the beginning of solidification of the wide sides of the casting. Also illustrated in this view is the position of the side dams on top of the raised ends of the dog-bone shaped rolls to contain the ends of the liquid metal pool.

FIG. 6 is a diagrammatical depiction of the casting system viewed from the end to illustrate both the wide side support below the rolls and the narrow end support that begins slightly below the nip between the outer diameters of the casting rolls.

FIG. 7 is a diagrammatical depiction of the casting system viewed from the wide side to illustrate the wide side support by the mold directly below the rolls, the continued support by rollers below the mold, and the mold end wall support members in position to both support and cool the narrow ends of the ultra-thin slab or thick-strip casting.

FIG. 8 is a diagrammatical depiction of the casting system viewed from the end to illustrate a means of separating two liquid metal pools in order to integrally cast two different metals or metal alloys to form a dual-layer ultra-thin slab or thick-strip casting.

FIG. 9 is a diagrammatical depiction of the casting system viewed from the end to illustrate a means of separating three liquid metal pools in order to integrally cast two or three different metals or metal alloys to form a multi-layer ultra-thin slab or thick-strip casting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 1, the ultra-thin slab or thick-strip 1 is partially solidified against a recessed casting surface 2 on each of two opposing, counter rotating rolls 3 and 4 and exits through the opening defined at the nip 5 by the recessed surfaces on the wide side and the inner edges of the recess on each roll, which form the narrow ends of the casting.

The maximum depth of the recess on each roll may be different. However, the two rolls preferably have recesses of the same maximum depth. In a preferred embodiment, the maximum depth of the recess on a casting roll is substantially one half of the casting thickness.

A side dam 6 is used at each end of the twin-roll assembly to contain a liquid pool of molten metal, which is continuously fed into the space between the upper portions of the two opposing rolls. Downward pressure exerted on side dam 6 forces it against the outer diameter 7 of each roll forming a seal to prevent metal leakage. The side dam 6 is preferably preheated before casting to prevent any of the incoming metal from solidifying against its inner surface thus limiting shell formation to the recessed casting surface 2 and the inner edges of the recessed areas.

As the cast shell reaches the nip 5, the two opposing edges of shell that solidified against the inner edges of the recess are brought together to form the end wall at each end of the casting, thus forming the perimeter of the casting. The point where they contact forms a continuous seam 8 between the two halves of the casting and continued solidification of the liquid core inside the hollow shell seals the seam 8 from the inside, preventing any leakage.

FIG. 2 illustrates a view through the center of the casting looking toward one end. The liquid metal pool 9 loses heat to the liquid-cooled rolls 3 and 4, causing the metal to crystallize or solidify against the cooled exposed surfaces it comes into contact with. As additional heat is transferred by conduction to the liquid-cooled rolls 3 and 4, the solidifying shell 10 of the wide side becomes thick enough at the nip 5 to span the short distance to the mold 11, below where additional heat is extracted and the shell becomes thicker still. The mold 11 is also liquid-cooled and consists of two wide side copper liners 12 and two narrow end copper liners 13 in FIG. 7 that support and cool the perimeter of the shell. The wide side copper liner is mounted to waterjacket 14 that internally directs the cooling liquid in and out of the copper liner 12 and provides structural support. The top of the mold is manufactured with a radius that matches up to the lower portion of the recessed casting surface 2 and is distanced from that surface by clearance gap 15. The top of the copper mold liners 12 & 13 have a small entry taper 16 to assure a smooth transition of the ultra-thin slab or thick-strip from the twin-roll section to the mold 11.

The initial point of solidification 17 for the wide side of the casting occurs near the top of the liquid metal pool 9, where it first contacts the recessed casting surface 2 on each roll. The solidifying shell 10 continues to grow thicker as it travels down through the machine until such time as it has completely solidified. Unlike thin-strip produced on twin-roll casters, which is completely solidified by time it passes through the nip 5 between the rolls, the ultra-thin slab or thick-strip has a liquid core 18 as it passes through the nip 5 and may have a liquid core 18 as it leaves the bottom of the mold 11. This is made possible by the support and continued cooling that occurs below the twin-roll section.

FIG. 3 is a view through the casting with the liquid metal removed and the rolls stopped. It illustrates how the ends of the ultra-thin slab or thick-strip 1 are formed against the inner edges of the recessed areas, which are referred to in this drawing as the narrow end casting surfaces 19 of the rolls 3 and 4. The initial point of solidification for each narrow end wall half 20 begins at the top of the liquid metal pool 9 shown on this drawing as height 21. No solidification occurs against the side dam 6 that presses against the outer diameter 7 of each roll. Thus the two narrow end wall halves 22 solidify against the narrow end casting surfaces 19 and are joined together at the nip 5 to connect the two halves of the casting at the seam 8 completing the perimeter of the ultra-thick slab or thick-strip 1.

Further illustrated in FIG. 3 is the solidifying shell 10 of the wide side against the recessed casting surface 2 and how it gains thickness as it travels down through the machine. Also shown is the solidifying shell 23 of the narrow end, and how the seam disappears from view slightly below the nip 5 as new metal solidifies against the inside surface and it too becomes thicker as it moves down through the machine.

FIG. 4 is a bottom view of the ultra-thin slab or thick-strip 1 exiting the nip between the dog bone-shaped twin-rolls 3 and 4. A radius 24 at the transition from the recessed casting surface 2 to the narrow end casting surface 19 results in the rounded corners 25 formed on the casting. The radius 24 is a size proportional to the design casting thickness, preferably not exceeding 50% of the casting thickness. The more preferred range for the radius 24 is between 0.1 and 0.33 of the casting thickness. But most preferably, the radius 24 is in a range from 0.2 to 0.3 of casting thickness. The angle 26 of that transition between the recessed casting surface 2 and the narrow end casting surface 19 that terminates at the outer diameter of the roll 7 must be greater than 90 degrees but less than 130 degrees to allow separation of the casting from rollers 3 and 4 immediately following exit of the casting through the roll nip 5 without impeding any drag forces upon the narrow ends of the cast product. Preferably the angle will range between 100 and 120 degrees.

For example if the design casting thickness is to be 20 mm thick, the radius in this corner would be no more than 25% of 20 mm or 5.0 mm. That leaves 10 mm of area between the radii for the seam connecting the two halves of the ultra-thin slab or thick strip casting. The corner radius may be as small as 10% of the design casting thickness leaving up to 80% of the area between the radii for the seam connecting the two halves of the ultra-thin slab or thick strip casting.

Further illustrated in FIG. 4 is the presence of the liquid core 18 between the solidifying shells 10 of the wide sides as the casting leaves the roll nip 5. Ordinarily this is not possible with twin-roll casting, because ferrostatic pressure exerted from the liquid pool of metal above would cause the shell to bulge outward from the center. But this system uses copper mold liners on both the wide sides 12 on FIG. 2 and the narrow ends 13 on FIG. 7 to provide continued shell support below the nip 5 of the twin rolls. As a result the wide side 27 of the casting and the narrow end 28 of the casting continue to grow in thickness as heat from the liquid core 18 is transferred into the liquid-cooled copper mold liners. In addition the seam 8 between the two halves of the narrow end of the casting 28 quickly becomes sealed from the inside as new metal from the liquid core 18 solidifies against the inner shell surfaces below the nip 5.

FIG. 5 is a top view above twin-rolls 3 and 4 showing the liquid metal pool 9 between the two rolls and the two side dams 6. The initial point of solidification 17 for the wide sides 27 of the casting begins at the top of the liquid metal pool 9 where it meets the recessed casting surface 2. The initial point of solidification for the narrow end wall half 20 begins where the liquid metal pool 9 meets the narrow end casting surface 19. The depth of the recessed casting surface 2, the radius 24 in the corner, and the angle 26 between the recessed casting surface 2 and the narrow end casting surface 19 that terminates at the outer diameter 7 of the roll dictates the shape of the narrow end wall halves 22 on FIG. 3 and the corners 25 on FIG. 4 of the casting. The angle 26 may be in a range that is substantially about 90° to about 130°, preferably from 100° to 120°.

FIG. 6 is an end view of the ultra-thin slab or thick-strip 1 exiting the bottom of the mold 11 illustrating the pointed shape of the narrow end mold water jacket 29 and the identical shape of the narrow end mold copper liner 13 in FIG. 7, which is shielded from view in this drawing. Also shown is the close proximity of the upper portion of the narrow end mold water jacket 29 to the outer diameter 7 of the rolls 3 and 4 and how they are separated only by a small narrow end clearance gap 30 below the nip 5. The width of the narrow end water jacket 29 and narrow end copper 13 in FIG. 7 is shown to be approximately the same as the distance between the recessed casting surfaces 2 of the two rolls. However they may become wider toward the back on very thin castings for added structural support. Also shown in this view are the wide side copper liners 12 of the mold and the wide side water jackets 14 of the mold on either side of the casting. Above the outer diameter 7 of the twin rolls 3 and 4 can be seen the side dam 6 that contains the liquid metal pool above the rolls.

FIG. 7 is a side view of the ultra-thin slab or thin-strip 1 casting coming through the mold 11 and entering a post-mold roll support section 31 below made up of support rolls 32 with bearing blocks 33 at each to allow the support rolls 32 to rotate while supporting the wide side of the casting 27 traveling out from the bottom of the mold 11. Between the mold and the first support roll 32 as well as between additional support rolls 32 is a gap 34 for spray water to access the surface of the casting to provide additional cooling and to promote the final solidification of the casting.

This view also shows the narrow end mold copper liner 13 attached to the narrow end water jacket 29 supporting the narrow end of the casting 28 as it passes through the mold 11 from just below the nip 5 to the bottom of the mold 11. This view also shows the back of the wide side mold water jacket 14 that has the wide side mold copper liner 12 on FIG. 2 mounted to the front supporting the wide side of the casting 27. The copper mold liners may have a casting surface that has a hard surface coating to minimize wear. The hard surface coating preferably has a hardness that is substantially within a range of about 250 to about 1200 Vickers, more preferably substantially within a range of about 500 to about 1200 Vickers and most preferably substantially within a range of about 800 to about 1200 Vickers.

With two wide side and two narrow ends supported, the entire perimeter of the casting is substantially supported by copper mold liners. A gap 34 at the corners between copper liners may be present as there is little need to support the rounded corner of the casting 25 which naturally benefits from two-dimensional cooling of the corners. Such a gap 24 could be right at the corner or slightly off the corner in either direction in the mold.

FIG. 8 is an end view of a dual-layer ultra-thin slab or thick-strip 35 being cast, with a separating dam 36 isolating the primary pool of molten metal 9 from a secondary liquid metal pool 37 of a different molten metal or different alloy of the same molten metal that forms a cladding 38 on one surface. This variation of the twin-roll casting process that is described in U.S. patent application Ser. No. 12/539,333 filed on 11 Aug. 2009 and its continuation Ser. No. 12/626,818, filed on 27 Nov. 2009, both of which are incorporated by reference as if set forth fully herein, results in a thinner layer of a second metal being formed on one side of the dual-layer ultra-thick slab of thick-strip 35. The location of the separating dam 36 can be varied along the contact area between roll 4 and the liquid metal pool 9, and the secondary liquid metal pool 37 to control the cladding 38 thickness. The closer the separating dam 38 is to the initial solidification point 17 the thinner the cladding 38 is on the final dual-layer ultra-thick slab or thick-strip casting 35. Additional solidification of the dual-layer casting after it passes the separating dam 36 occurs from the primary liquid metal pool 9 beginning at the secondary initial point of solidification 39 on that side of the casting.

The thickness of each side of the dual-layer metal is preferably substantially within a range of about 1:2 to about 1:40, more preferably substantially within a range of about 1:3 to about 1:35 and most preferably substantially within a range of about 1:4 to about 1:30. In other words, 1 mm of one metal alloy clad onto 40 mm of a second metal alloy could be used to cast a clad dual-layer ultra-thin slab or thick strip.

The ratio of cladding 38 thickness to the remainder of the dual-layer ultra-thin slab or thick-strip 35 thickness formed from the primary liquid metal pool 9 can be varied from 1:2 to 1:40 by varying the location of the separating dam 36 position and varying the gap between the two rolls 3 and 4.

The casting process could also be altered by positioning a separating dam over each of the two rolls to form three liquid metal pools for casting a multi-layer metal. This would produce thickness ratios between three layers from about 1:1:1 to as high as 1:40:1 whereby the center of the multi-layer metal alloy would generally constitute the majority of the thickness of the multi-layer ultra-thin slab or thick strip. One example of using the process would be to cast a thin cladding or stainless steel layer on each side of a carbon steel core thus getting the benefit of the corrosion-resistant stainless steel on each surface with the cost and strength benefits of having the less expensive and stronger carbon steel in the center of the multi-layer ultra-thin slab or thick strip. Preferably, an austenitic stainless steel in the 300 series would be used for maximum corrosion resistance. The outer layer could alternatively be a material such as zinc or copper. As another alternative, the core layer could be a lightweight metal such as aluminum, while the cladding layer is a material such as carbon steel or stainless steel.

FIG. 9 is an end view of a multi-layer ultra-thin slab or thick-strip 40 being cast with separating dams 36 isolating the primary pool of molten metal 9 from a secondary liquid metal pool 37 and a third liquid metal pool 41 of different molten metals or different alloys of the same molten metal that form cladding 38 on one side of the casting from the secondary liquid metal pool 37 and cladding 42 on the opposite side of the casting from the third liquid metal pool 41. Details of this embodiment are the same as those described in FIG. 8 but applied to both sides of the multi-layer ultra-thin slab or thick-strip 40 casting to form cladding 38 on one side and cladding 42 on the other side of a multi-layer ultra-thin slab or thick-strip 40.

The ratio of cladding thicknesses to the remainder of the multi-layer ultra-thin slab or thick-strip 40 thickness formed from the primary liquid metal pool 9 can be varied by changing the location of the separating dam 36 positions as described in FIG. 8 and the gaps between the two rolls 3 and 4. This would produce thickness ratios from 1:1:1 to as high as 1:40:1 whereby the center of the multi-layer metal alloy formed from the primary liquid metal pool 9 would generally constitute the majority of the thickness of the multi-layer ultra-thin slab or thick strip 40. In other words, the ratio of the thickness of the center core layer to the thickness of either or both of the outer layers could be substantially within a range of about 1:1 to about 40:1. More preferably, such a ratio would be substantially within a range of about 1:3 to about 1:35 and most preferably substantially within a range of about 1:4 to about 1:30.

By providing a means to solidify not only the side walls of the casting in the twin-roll process but the narrow end walls as well, the full perimeter of the ultra-thin slab or thick-strip will be formed thus forming a full shell around a liquid core of molten metal. This will enable casting speeds to range from 6 meters per minute to as high as 100 meters per minute depending on the final thickness of the product. Preferably the casting speeds will range from 8 meters per minute casting 35 mm thick ultra-thin slabs to 80 meters per minute casting 5 mm thick strip.

The system in the present invention may be used to make a variety of steel or alloy products. For example, the system shown in FIG. 9 is capable of making three layer ultra-thin slab or thick strip 40 that is suitable for applications where steel corrosion is a significant concern, such as pipelines. Corrosion of steel always occurs through oxidation at its surface where the carbon steel comes into direct contact with oxygen. One common solution to steel corrosion is using stainless steel where large amounts of very expensive metals such as nickel and chrome are pre-alloyed into the steel. The problem with stainless steel is its high cost, which makes it cost-prohibitive and impractical for most applications. In addition, stainless steel tends to have less strength than other steels so it may not be suitable for applications that normally require high-strength steels.

The system in FIG. 9 may be used to make steel that has a stainless steel cladding 42 yet with normal steel core. Such “hybrid” steel will have a layer of stainless steel on the surface to resist corrosion and a core of low-cost or high-strength steel. Such hybrid steel is an ideal material to provide high strength support in an environment in which steel corrosion is a significant concern, such as pipelines.

An ultra-thin slab or thick strip produced according to the invention could also be used to produce a multilayer material such as armor for military vehicles. In this embodiment, the first layer may be made up of a material such as a hard, high carbon steel with superior strength, while the second layer may be a softer, low carbon steel.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

1. A twin roll continuous casting system, comprising: first and second casting rolls; and a recessed roller surface on at least one of the first and second casting rolls for forming at least a portion of a casting shell.
 2. A twin roll continuous casting system according to claim 1, wherein a portion of the recessed roller surface is constructed and arranged to facilitate narrow end solidification of a casting shell.
 3. A twin roll continuous casting system according to claim 1, wherein both the first and second casting rolls have a recessed roller surface.
 4. A twin roll continuous casting system according to claim 3, wherein the recess defined in the first casting roll has a first maximum depth and the recess defined in the second casting roll has a second maximum depth, and wherein the first maximum depth is substantially the same as a second maximum depth.
 5. A twin roll continuous casting system according to claim 3, wherein the recess defined in the first casting roll is substantially symmetrical with respect to the recess that is defined in the second casting roll.
 6. A twin roll continuous casting system according to claim 1, wherein the recessed roller surface is constructed and arranged to form a wide side of a casting shell.
 7. A twin roll continuous casting system according to claim 1, wherein the recessed roller surface has a maximum depth, and wherein the maximum depth is substantially equal to about 50% of a design casting thickness.
 8. A twin roll continuous casting system according to claim 1, wherein the recessed roller surface includes a recessed casting surface and a narrow end casting surface, and wherein a recess angle defined between the recessed casting surface and the narrow end casting surface is substantially within a range that is about 90° to 130°.
 9. A twin roll continuous casting system according to claim 8, wherein the recess angle is substantially within a range of about 100° to 120°.
 10. A twin roll continuous casting system according to claim 1, wherein the recessed roller surface includes a recessed casting surface and a first narrow end casting surface, and wherein a corner defined between the two casting surfaces is concavely radiused.
 11. A twin roll continuous casting system according to claim 10, wherein the recessed roller surface includes a second narrow end casting surface, and wherein the second narrow end casting surface is substantially symmetrical in shape to the first narrow end casting surface.
 12. A twin roll continuous casting system according to claim 10, wherein the concavely radiused corner defines an average radius of curvature, and wherein a ratio of the average radius of curvature to a design casting thickness is substantially no more than about 0.5 or less.
 13. A twin roll continuous casting system according to claim 12, wherein the ratio of the average radius of curvature to the design casting thickness is substantially within a range of about 0.1 to about 0.33.
 14. A twin roll continuous casting system according to claim 13, wherein the ratio of the average radius of curvature to the design casting thickness is substantially within a range of about 0.2 to about 0.3.
 15. A twin roll continuous casting system according to claim 1, further comprising support structure positioned beneath the casting rolls for supporting the casting shell.
 16. A twin roll continuous casting system according to claim 15, wherein the support structure is constructed and arranged to support substantially an entire perimeter of the casting shell as viewed in transverse cross-section.
 17. A method of continuously casting a metal material, comprising steps of: providing a molten metal material; and cooling the molten metal material into a solid casting using a casting roller that has a recess defined therein.
 18. A method of continuously casting a metal material according to claim 17, wherein the speed of casting is substantially between about 6 meters per minute to about 100 meters per minute.
 19. A method of continuously casting a metal material according to claim 18, wherein said speed of casting is substantially between about 8 meters per minute and 80 meters per minute.
 20. A twin roll continuous casting system, comprising: first and second casting rolls that are constructed and arranged to form a casting shell; and support structure positioned beneath the casting rolls for supporting substantially an entire perimeter of the casting shell as viewed in transverse cross-section.
 21. A twin roll continuous casting system according to claim 20, wherein said support structure comprises copper.
 22. A twin roll continuous casting system according to claim 20, wherein said support structure is liquid cooled.
 23. A twin roll continuous casting system according to claim 20, wherein the support structure is constructed and arranged to facilitate continued solidification of the casting shell.
 24. A twin roll continuous casting system according to claim 20, wherein the support structure includes a hard surface coating to minimize wear, the hard surface coating having a hardness that is substantially within a range of about 250 to about 1200 Vickers.
 25. A twin roll continuous casting system according to claim 20, wherein the support structure includes a first support surface for supporting a narrow end of the casting shell, and wherein the first support surface is tapered at an upper portion thereof.
 26. A twin roll continuous casting system according to claim 25, wherein portions of the first support surface beneath the upper portion are substantially flat.
 27. A twin roll continuous casting system, comprising: a cavity that is constructed and arranged to hold a liquid metal pool; first and second casting rolls; at least two separating dams positioned axially within the cavity for separating the cavity into at least three separate metal pools; wherein different materials may be cast simultaneously in at least three layers into a single casting shell.
 28. A twin roll continuous casting system according to claim 27, wherein at least one of the at least three metal pools includes a surface cladding material.
 29. A twin roll continuous casting system according to claim 27, wherein a thickness ratio between about 1:1:1 to as high as 1:40:1.
 30. A method of continuously casting a material, comprising steps of: providing at least three different pools of molten material within a cavity; and fabricating a multi-layer casting shell using first and second casting rolls simultaneously using material from each of the at least three different pools of molten material.
 31. A method of continuously casting material according to claim 30, wherein the step of fabricating a multilayer casting shell includes fabricating at least one cladding layer from a cladding material.
 32. A method of continuously casting material according to claim 31, wherein the cladding material is selected from a group consisting of stainless steel, zinc and copper.
 33. A twin roll continuous casting system, comprising: a cavity that is constructed and arranged to hold a liquid metal pool; at least one separating dam positioned within the cavity for separating the cavity into at least two separate metal pools of different materials; and first and second casting rolls for receiving material from the at least two separate metal pools and casting at least a first layer having a first thickness and a second layer having a second thickness into a single casting shell, and wherein a ratio of the first and second thicknesses is substantially within a range of about 1:2 to about 1:40.
 34. A twin roll continuous casting system according to claim 33, wherein a thickness ratio between two of the layers is substantially within a range of about 1:3 to about 1:35.
 35. A twin roll continuous casting system according to claim 34, wherein a thickness ratio between two of the layers is and most preferably substantially within a range of about 1:4 to about 1:30. 